In an attempt to increase capacity of wireless communications, frequency bands used are increasingly in a broader frequency range as well as in a higher frequency region. In recent years, not only a microwave band (not less than 0.3 GHz and not more than 30 GHz) but also a millimeter wave band (not less than 30 GHz and not more than 300 GHz) is used in wireless communications. In particular, 60 GHz band, in which a great attenuation occurs in the atmosphere, is attracting attention as a band in which data leakage is less likely to occur and a large number of communication cells with reduced size can be arranged.
An antenna which is used in a wireless communication in 60 GHz band is expected to have a high gain as well as operate in a wide frequency band. This is because a great attenuation occurs in 60 GHz band in the atmosphere, as described above. Examples of an antenna which has a gain high enough to allow the antenna to be used in 60 GHz band include an array antenna. Note here that the array antenna refers to an antenna in which a plurality of antenna elements are arranged in an array or in matrix.
In the array antenna, a main beam direction of a radiated electromagnetic wave, which is obtained by superimposing electromagnetic waves radiated from the respective plurality of antenna elements, can be changed by control of a time delay imparted to a radio frequency signal supplied to each of the plurality of antenna elements. The array antenna having such a beam forming function is called a phased array antenna, and has been a subject of vigorous research and development.
A principle of beam forming performed in the phased array antenna is discussed below with reference to FIG. 12. The following description is based on the assumption that a plurality of antenna elements A1 through An constituting a phased array antenna are arranged on a specific straight line at regular intervals d.
In a case where radio frequency signals having an identical phase are supplied to the respective antenna elements A1 through An, an equiphase plane parallel to the specific straight line is formed, and a main beam direction is perpendicular to the equiphase plane. In this case, when time delays δ1 through δn with an equal difference therebetween are imparted to the respective radio frequency signals supplied to the antenna elements A1 through An, the equiphase plane is tilted in accordance with a time delay difference Δt=δ2−δ1=δ3−δ2= . . . =δn−δn−1. Here, the time delay difference Δt and a tilt angle (an angle between the specific straight line and the equiphase plane) α of the equiphase plane are in the following relation (c is a light speed in vacuum).Δt−d×sin α/c 
Accordingly, in a case where a time delay Si imparted to a radio frequency signal supplied to each antenna element Ai is controlled so as to increase the time delay difference Δt, it is possible to increase the tilt angle α. In an opposite case where the time delay Si imparted to the radio frequency signal supplied to each antenna element Ai is controlled so as to reduce the time delay difference Δt, it is possible to reduce the tilt angle α. Thus described is the principle of beam forming.
The following description will discuss, with reference to FIGS. 13 through 15, typical configurations of conventional phased array antennas. A phased array antenna 13 shown in FIG. 13 is a transmitting antenna. A phased array antenna 14 shown in FIG. 14 is a receiving antenna. A phased array antenna 15 shown in FIG. 15 is a transmitting and receiving antenna. Hereinafter, a time delay is simply referred to as a “delay.”
The phased array antenna 13 shown in FIG. 13 (1) uses time delay elements TD11 through TD1n so as to impart delays δ1 through δn with an equal difference therebetween to a radio frequency signal VRF(t) externally supplied and (2) supplies delayed radio frequency signals VRF(t−δ1) through VRF(t−δn) thus obtained to antenna elements A1 through An. In a case where the delays δ1 through δn imparted to the radio frequency signal VRF(t) are set so that a time delay difference Δt=δ2−δ1=δ3−δ2= . . . =δn−δn−1 coincides with d×sin α/c, it is possible to transmit efficiently an electromagnetic wave having a tilt angle of α of an equiphase plane.
The phased array antenna 14 shown in FIG. 14 (1) uses time delay elements TD21 through TD2n so as to impart delays δ1 through δn with an equal difference therebetween to respective radio frequency signals VRF(t+δ1) through VRF(t+δn) outputted from antenna elements A1 through An and (2) outputs, outside the phased array antenna 14, a delayed radio frequency signal VRF(t) thus obtained. In a case where the delays δ1 through δn imparted to the radio frequency signals VRF(t+δ1) through VRF(t+Sn) are set so that a time delay difference Δt−δ2−δ1−δ3−δ2= . . . =δn−δn−1 coincides with d×sin α/c, it is possible to receive efficiently an electromagnetic wave having a tilt angle of α of an equiphase plane.
The phased array antenna 15 shown in FIG. 15 is obtained by combining the phased array antenna 13 shown in FIG. 13 and the phased array antenna 14 shown in FIG. 14 with use of circulators (diplexers or switches) C1 through Cn. Each antenna element Ai is for both transmission and reception. Each circulator Ci is an element which (i) has three or more ports to and from which a signal is supplied and outputted and (i) outputs a signal, which is supplied to a certain port, through a port subsequent to the certain port along a direction indicated by a curved arrow shown in FIG. 15. In the phased array antenna 15, each circulator Ci has a function of (i) supplying, to a corresponding antenna element Ai, a delayed radio frequency signal VRF(t−δi) outputted from a corresponding time delay element TD1i for transmission and (ii) supplying, to a corresponding time delay element TD2i for reception, a radio frequency signal VRF(t+δi) outputted from the antenna element Ai. In the case of the diplexers or switches, each diplexer or switch has a function identical to the above function.
However, the phased array antennas 13 through 15 shown in FIGS. 13 through 15 are not suitable for use in a millimeter wave band. This is because it is difficult to impart a highly accurate delay to a radio frequency signal in a millimeter wave band with use of electrical means such as a time delay element.
In regard to this, there is also known a phased array antenna which delays a radio frequency signal with use of optical means. This phased array antenna, however, requires use of an optical component which is more expensive than an electronic component, so that an increase in cost is inevitable. Especially in a case where the phased array antenna is assumed to be used in a millimeter wave band, it is necessary to use a highly expensive modulator, photoelectric conversion element, and the like, by which a significant increase in cost is expected.
In view of this, in order for a phased array antenna usable in a millimeter wave band to be provided without use of optical means, it is an option to employ, in place of a time delay device that delays a radio frequency signal, a time delay device that delays an intermediate frequency signal or a local signal, each of which has a frequency lower than that of the radio frequency signal. Examples of such a time delay device are disclosed in Patent Literature 1 and Non-patent Literature 1.